Ok but mtbl. There's a whole culture devoted to immutable (write once) databases at very large scale. There's another whole culture which thinks ACID shouldn't

Ok but mtbl. There's a whole culture devoted to immutable (write once) databases at very large scale. There's another whole culture which thinks ACID shouldn't apply to distributed systems and that global context should be avoided. In the context of cloud, (MariaDB versus MySQL) what both brand options offer is multiple engine support. The cloud tends to offer its own engines, under its own brand(s).